Back pain at 18 weeks is pretty common during pregnancy, but since you're getting it often, best to mention it to your doctor just to make sure everything's okay, could be ligament stretching or posture changes, but they'll want to check. A few of us mums had similar pain and found it helped to wear a maternity support belt, adjust how we sit at work, and do some gentle stretches, but you really need your doc to rule out anything else first, especially if it's persistent like yours.